1. Field of the Invention
The present invention relates to a method of detecting subnodes of an optical switch for switching wavelength division multiplexes. Wavelength division multiplexes are used to transmit data in optical fiber networks. The invention applies to optical wavelength division multiplex transmission networks and to equipment units of such networks, such as optical switches.
2. Description of the Prior Art
At present, data is transmitted in optical networks on light beams corresponding to a plurality of wavelengths, with a bit rate of the order of 10 Gbit/s (109 bit/s). The light beams are transmitted by optical fibers. To route data across the network it is necessary to provide wavelength division multiplex switching blocks. The technology of these switches enables the incoming signal to be switched directly in its optical form. There is no need to transform light signals into electrical signals in order to switch them. The optical switches receive at their input a number of optical fibers each corresponding to a plurality of wavelengths. On reaching the optical switch, the light signal is wavelength division demultiplexed and each wavelength then arrives a port of the optical switch that switches it to an outgoing fiber. The wavelengths are remultiplexed at the output of the switch. The switches or switching nodes are therefore “monoblock” nodes in the sense that they can switch only wavelengths. These switching blocks must therefore be allocated as many input and output ports as there are wavelengths to be switched.
There is at this time a very considerable expansion of telecommunications, reflected in increased requirements for data transmission. Optical fiber transmission has not escaped this phenomenon and the quantity of data transmitted by optical networks has increased considerably. From now on, optical fibers will have to be designed to transmit more and more wavelengths. It is currently possible to transmit up to 256 wavelengths per optical fiber.
The switching nodes in optical networks receive more and more fibers and therefore more and more wavelengths to be switched. To give a concrete example, an optical switch might have to switch 100 fibers each carrying 160 wavelengths, each wavelength providing a data bit rate of 10 Gbit/s (109 bit/s). The total bit rate to be switched is then 160 Tbit/s (1012 bit/s). The total number of incoming wavelengths is 16 000. To switch all the wavelengths individually, it would be necessary to design an optical switch with a very large number (16 000×16 000) of input and output ports, which is not feasible with present-day optical switching matrices. Controlling such matrices electrically would become too complex because of the very large number of switching points.
With nodes of the above size, one might imagine that some of the traffic would come from the same direction and go in the same direction. One solution to the problem of reducing the number of ports is therefore to group the wavelengths into bands and to switch them together, so that a portion of the traffic could be switched simultaneously by using a single port for a plurality of wavelengths.
More generally, to reduce the number of ports, one might wish to switch a portion of the total traffic at fiber level, another portion at band level and a final portion at wavelength level. Switching complete wavelength division multiplexes, i.e. switching fibers, bands of wavelengths and wavelengths, in the same optical node corresponds to different switching capacities. In this configuration, planning wavelength division multiplex networks is based on optical switches which have a “multigranularity” architecture.
The “granularity” concept relates to predefined sets of transmission resources (typically carrier wavelengths or wavelength division multiplexes) where the resources of a set can be treated as a whole for some common processing (typically switching). A “multigranularity” architecture therefore takes account of different levels of granularity for switching the total traffic of a switch. For example, a portion of the total traffic might be switched at the “fiber” level, i.e. by grouping all of the wavelengths that can be conveyed by an optical fiber, which thus corresponds to the highest level of granularity. Another portion might be switched at the band of wavelengths level, which corresponds to an intermediate level of granularity. A final portion might be switched at the wavelength level, which corresponds to the lowest level of granularity. Further intermediate levels of granularity can be defined.
FIG. 1 is a block diagram of a prior art optical switching node with a multigranularity architecture. The multigranularity architecture has led to a change from monoblock switching nodes to switching nodes consisting of a stack of subnodes. Each switching subnode is defined by a level of granularity. Thus the figure shows a switching subnode FXC at the fiber level of granularity, a switching subnode BXC at the band level of granularity, and a switching subnode WXC at the wavelength level of granularity.
In FIG. 1, the incoming fibers IF are first routed to the input ports IP of the switching subnode FXC. A few of the incoming fibers IF are switched directly to the output fibers OF via the output ports OP of the switching subnode FXC. A fiber AF coming from the client is directly inserted at a fiber insertion port Pins of the switching subnode FXC. A fiber DF sent to the client is extracted from a fiber extraction port Pext of the subnode FXC. The fiber DF must be wavelength division demultiplexed for the client, but the demultiplexers are not shown in the figure. Fibers Fbf are inserted from the switching subnode BXC to the fiber insertion ports Pins of the subnode FXC. These fibers Fbf come from the band to fiber multiplexer Mux B→F which multiplexes the bands coming from the output ports OP of the switching subnode BXC. Finally, fibers Ffb are extracted from the subnode FXC via extraction ports and are sent to the input ports IP of the subnode BXC after the fibers are demultiplexed into bands in the fiber to band demultiplexer Demux F→B.
The same switching process is used at the next lower level of granularity, i.e. in the switching subnode BXC at the band level of granularity, as well as at the lowest level of granularity, i.e. in the switching subnode WXC at the wavelength level of granularity.
A few of the bands arriving at the input ports IP of the subnode BXC are switched to the output ports OP of the subnode BXC. A band AB coming from the client is directly inserted at an insertion port of the subnode BXC. A band DB sent to the client is extracted via an extraction port Pext of the subnode BXC. The band DB must be wavelength division demultiplexed for the client, but the demultiplexers are not shown in the figure. Bands Bλb are inserted from the switching subnode WXC at the insertion ports Pins of the subnode BXC. These bands Bλb come from the multiplexer Mux λ→B which multiplexes wavelengths from the output ports OP of the switching subnode WXC into bands. Finally, bands Bbλ are extracted from the subnode BXC via extraction ports and are sent to the input ports IP of the subnode WXC after the bands are demultiplexed into wavelengths in the band to wavelength demultiplexer Demux B→λ.
The same switching process is used again in the subnode WXC. A few of the wavelengths arriving at the input ports IP of the subnode WXC are switched to the output ports OP of the subnode WXC. Wavelengths Aλ coming from the client are directly inserted at insertion ports Pins of the subnode WXC. Wavelengths Dλ sent to the client are extracted via extraction ports of the subnode WXC.
The network is planned without any specification at the level of the nodes. Traffic routing is based on an algorithm that yields the shortest path in terms of distance. Resources are allocated to the network at the wavelength level. Although planning is based only on the wavelength level of granularity, a great part of the traffic at each optical switching node can be processed at the fiber and band levels, as if this were “natural”. At a switching node, either the whole of the multiplex or only a portion of the multiplex comes from the same incoming fiber and goes to the same outgoing fiber. The problem is precisely to quantify the portions of the traffic that must be switched at the fiber, band and wavelength levels of granularity.
Accordingly, the technical problem that arises is that of finding a means of detecting the switching subnodes actually required and therefore of providing a method of subdividing the switching matrix of the initial node.
One object of the present invention is specifically to provide a method of detecting subnodes in an optical switching node so that it is possible to determine the traffic at and the size of each subnode. The method according to the invention therefore optimizes the number of ports needed at an optical switching node and thereby optimizes switching costs by using the highest levels of granularity for switching whenever possible.
To this end, the invention starts from the initial switching matrix of the monoblock node whose subnodes are to be detected. For each subnode, the invention then selects the fibers, bands of wavelengths or wavelengths complying with the switching constraints corresponding to that subnode. Which portion of the traffic at each node can be processed in such a subnode is detected. The switching constraints are translated into incoming fiber/outgoing fiber terms and more generally into incoming granularity/outgoing granularity terms, as well as into wavelength translation/band translation terms. The detection method is implemented by an algorithm defining all the necessary detection steps.